1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, which is provided with a function of forming an image on a recording material such as a sheet.
2. Description of the Related Art
Recently, as an image forming apparatus such as a printer and a copying machine using an electrophotographic method, there is widely used a tandem type image forming apparatus capable of forming a color image on various kinds of recording materials at high speed with high quality.
For example, the tandem type image forming apparatus has a structure in which four image forming units for yellow (Y), magenta (M), cyan (C), and black (K) are arranged in parallel with each other, and an intermediate transfer belt is circulated so as to be sequentially brought into contact with photosensitive drums of the image forming units. Then, onto the circulating intermediate transfer belt, toner images of yellow, magenta, cyan, and black colors are sequentially formed by the image forming units, and primarily transferred thereonto in a multiple transfer manner. After that, the toner images are collectively secondarily transferred onto a recording material by the intermediate transfer belt and a secondary transfer member. Finally, the toner images on the recording material are fixed, whereby the color image is formed.
Generally, in the image forming apparatus capable of forming a multiple color image, a multiple color mode in which all the image forming units function and a single color mode in which only one of the image forming units functions can be selectively carried out.
In the above-mentioned tandem type image forming apparatus capable of switching between the multiple color mode and the single color mode, if the photosensitive drums of the image forming units, which do not function at the time of image formation in the single color mode, continue to be in contact with the intermediate transfer belt, there is a fear that the following problem arises. That is, surfaces of the photosensitive drums are worn due to contact with the intermediate transfer belt.
Thus, Japanese Patent Application Laid-Open No. 2003-337454 proposes a structure in which the photosensitive drums of the image forming units, which do not function in the single color mode, are separated from the intermediate transfer belt. FIGS. 8A and 8B are schematic sectional views of the structure of the image forming apparatus. FIG. 8A illustrates a state when image formation is performed in the multiple color mode, and FIG. 8B illustrates a state when image formation is performed in the single color mode. In the multiple color mode, as illustrated in FIG. 8A, the image formation is performed on a recording material S in a state in which all photosensitive drums 103a, 103b, 103c, and 103d are brought into contact with an intermediate transfer belt 108. In the single color mode, as illustrated in FIG. 8B, the image formation is performed on the recording material S in a state in which only the photosensitive drum 103d is brought into contact with the intermediate transfer belt 108.
Further, Japanese Patent No. 3,943,758 proposes an image forming apparatus which conveys the recording material while the recording material is attracted to an electrostatic conveying belt, and transfers the toner images formed on the photosensitive drums to the recording material to thereby perform recording. The photosensitive drums of the image forming units, which do not function in the single color mode, are separated from the electrostatic conveying belt.
However, in a tandem color image forming apparatus capable of switching a position of the intermediate transfer belt between the multiple color mode and the single color mode, a restraint force of the intermediate transfer belt is weakened in the single color mode. Thus, there is a fear that an image formation defect is generated.
That is, in the multiple color mode, the intermediate transfer belt is sandwiched between the photosensitive drums of the multiple image forming units and transfer rollers, and is restrained at many portions (points) to stably circulate and move therearound. Meanwhile, in the single color mode, the intermediate transfer belt is sandwiched only at one point between a photosensitive drum of a black image forming unit and a transfer roller, and the points for restraining the intermediate transfer belt are reduced. Thus, it becomes easy to generate fluctuation in circulating speed of the intermediate transfer belt.
For example, when the recording material having high rigidity such as a thick sheet proceeds onto the intermediate transfer belt, fluctuation in speed of the intermediate transfer belt is generated due to impact. In this case, if the image is transferred onto the intermediate transfer belt from the photosensitive drum of the black image forming unit, density of the toner image is changed, and hence there is a risk that disorder of the image is generated at the portion thereof.